At the end of level 1-2, run to the pipe at the end and jump on it. Knock out the two blocks just to the left of the one straight above. Stand on the very edge of the tube, then crouch and jump into the wall as shown. Once you've slid through, climb down the first pipe to reach World -1, a never ending water level.
There is no way to exit World -1; even if you die, you will begin again in -1. Your only choice is to either continue playing, or restart the game.